CODEWITHSUNDEEP
phpregexrecursion
Prog
Prog

Reputation: 3

recursive regular expression to process nested strings enclosed by {| and |}

In a project I have a text with patterns like that:

{| text {| text |} text |}
more text

I want to get the first part with brackets. For this I use preg_match recursively. The following code works fine already:

preg_match('/\{((?>[^\{\}]+)|(?R))*\}/x',$text,$matches);

But if I add the symbol "|", I got an empty result and I don't know why:

preg_match('/\{\|((?>[^\{\}]+)|(?R))*\|\}/x',$text,$matches);

I can't use the first solution because in the text something like { text } can also exist. Can somebody tell me what I do wrong here? Thx

Upvotes: 0

Views: 2694

Answers (3)

outis
outis

Reputation: 77420

You've got a few suggestions for working regular expressions, but if you're wondering why your original regexp failed, read on. The problem lies when it comes time to match a closing "|}" tag. The (?>[^{}]+) (or [^{}]++) sub expression will match the "|", causing the |} sub expression to fail. With no backtracking in the sub expression, there's no way to recover from the failed match.

Upvotes: 1

Alan Moore
Alan Moore

Reputation: 75242

Try this:

'/(?s)\{\|(?:(?:(?!\{\||\|\}).)++|(?R))*\|\}/'

In your original regex you use the character class [^{}] to match anything except a delimiter. That's fine when the delimiters are only one character, but yours are two characters. To not-match a multi-character sequence you need something this: 

(?:(?!\{\||\|\}).)++

The dot matches any character (including newlines, thank to the (?s)), but only after the lookahead has determined that it's not part of a {| or |} sequence. I also dropped your atomic group ((?>...)) and replaced it with a possessive quantifier (++) to reduce clutter. But you should definitely use one or the other in that part of the regex to prevent catastrophic backtracking.

Upvotes: 3

Tor Valamo
Tor Valamo

Reputation: 33769

See PHP - help with my REGEX-based recursive function

To adapt it to your use

preg_match_all('/\{\|(?:^(\{\||\|\})|(?R))*\|\}/', $text, $matches);

Upvotes: 0

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